1. Field of the Invention
This invention relates generally to electric vehicles, and more particularly to controls for AC adjustable speed drives.
2. Description of the Related Art
Conventional electric vehicles are designed to achieve high efficiency. For example, low loss tires are used to reduce rolling friction, and single speed transmissions are chosen over multiple speed automatic transmissions. Conventional electric motors operate at high speeds and therefore require large numeric gear reductions. The electric drives required for high performance electric vehicles have large starting torque and high power.
Vehicle power trains exhibit driveline oscillations that occur at the eigen frequencies (characteristic frequencies of the mechanical systems) and multiples of the eigen frequencies of mechanical vehicle components such as gears, shafts, constant velocity joints, motor mounts, and vehicle suspension systems. Driveline mechanical natural frequencies for cars and small trucks typically occur in the range of four hertz to seven hertz. Electric drive systems, especially AC drive systems, often produce small torque pulsations that are related to the electrical excitation frequency. At certain motor speed and torque operating points, the electrical excitation frequency, and therefore the torque pulsation frequency, aligns with the eigen frequencies of the mechanical system and produces a sustained, self-excited driveline oscillation that will hereinafter be referred to as torque shudder.
During low speed operations such as one to three miles per hour in a parking lot, efficiency improvement factors require the drive to operate at torque levels of one to three percent of its maximum rated torque. While operating the drive at one percent of rated maximum torque and two to three percent of maximum speed, it is difficult to avoid producing torque pulsations on the order of .+-.0.5 lb-ft that would sustain torque shudder.
Even if a drive could be tuned to suppress torque pulsation below the .+-.0.5 lb-ft level, it is unlikely that drives with component and parameter variations could be mass-produced in large quantities that would maintain this exceptional torque smoothness because precision fabrication of such drives would be difficult due to offsets, errors, and temperature variations which create problems such that each drive would need to be specially calibrated.